Some of the displacement controlled hydraulic proportional valves have a main control valve for controlling the main fluid flow between a supply pump and a hydraulic actuator and a pilot valve for controlling actuation of the main control valve. The pilot valve is typically controlled by a proportional solenoid exerting a control force on a pilot valve spool to move the pilot valve spool relative to the main valve spool. Moving the pilot valve spool toward the main valve spool controls fluid pressure in a control chamber such that the main spool moves toward the pilot valve spool. Displacement of the main valve spool is mechanically fed back to the pilot valve spool through a force feedback spring positioned between the valve spools so that displacement of the main valve spool is proportional to the control force exerted on the pilot valve spool by the solenoid.
One of the problems encountered with such displacement controlled control valves is caused by friction on the pilot valve spool due to side loading thereon. The friction causes sticking of the pilot valve spool resulting in reduced accuracy of the positioning of the pilot valve spool which ultimately reduces the position accuracy of the actuator. In severe cases, the actuator may have a tendency to hunt for position. One factor that contributes to the side loading is that the force feedback spring is normally a coil compression spring which has a tendency to buckle under compression. Heretofore, one end of the feedback spring has been seated on a spring retainer fastened to the pilot valve spool. Buckling of the spring causes the spring retainer to impart a twisting moment onto the pilot valve spool resulting in side loading on the opposite ends of the spool. In severe cases, the metering lands on the spool tend to hang up on the edges of the annuluses of the valve body.
Another problem encountered with such displacement controlled control valves is that the main spool has a tendency to oscillate or become unstable under some operating conditions. One of the factors contributing to the instability is the fact that displacement of the main control valve spool is dependant upon the control force and feedback force on the pilot valve spool reaching equilibrium. Thus, if the acceleration forces on the main spool are too high, the main spool tends to overshoot the desired position such that the feedback forces acting on the pilot valve causes the pilot valve spool to oscillate which, in turn, causes the main valve spool to oscillate. The same pilot valve is frequently used with several sizes of main control valves and the instability is more pronounced on the control valves having smaller diameter main valve spools.
Thus, it would be advantageous to have a displacement controlled hydraulic proportional control valve designed to minimize side loading on the pilot valve spool to maximize the position accuracy of the pilot valve spool and, thus, the position accuracy of the actuator controlled by the control valve. It would also be advantageous to have a displacement controlled hydraulic proportional control valve which is stable regardless of the size of the main control valve spool and reduces the potential for hunting movements.
The present invention is directed to overcoming one or more of the disadvantages or problems as set forth above.